Generally, a sirocco fan includes, as shown in FIG. 1, a fan 1 having blades 2 around the rotational axis of the fan, the blades 2 being formed to curve forward relative to the direction of rotation R.
Air flows through the blade 2 from leading edge A to trailing edge B along the contour of the blade.
In this time, the air flow within the chord C slopes down deeply according to the contour of the chord and the air flow outside of the chord D can not be continuous inspire of the inertia of air flow, but separate from the blade 2.
This flow separation results in a decrease in speed and a reduction in efficiency due to the formation of drag.
The existence of wake zone downstream of the cutting 2a formed at the outer end of the blade 2 causes noise by the small vortex formed therein.